Answers to “how do switches work by tracing packet flow” post

So just to get you up to speed. This post will give the answers to my last post. In the last post we had PC1 ping PC4. ARP ran and helped PC1 and PC4 talk to each other.

The questions I left you with were:

1.) What would happen if PC2 were to ping PC5? Would ARP be needed? If so what devices would get the ARP request.

2.) What would happen after PC2 pinged PC5 and PC1 would ping PC5? Would ARP be needed? If so what devices would get the ARP request?

3.) What would happen after PC1 pinged PC5 and PC1 pinged PC6? Would ARP be needed? If so what devices would get the ARP request?

4.) Would would happen after PC1 pinged PC6 and PC1 pinged PC3? Would ARP be needed? If so what devices would get the ARP request?

Answers:

1.) Well neither PC2 nor PC5 have talked on the network. Because of this I would suspect that neither of the switches nor any of the other computers on the network know what their MAC address were. Because of this ARP would be needed and the process would be similar to what you saw for PC1 pinging PC4.

I’m not going to do the print out again but one thing to note is even though switch1 even though it knows the MAC address of PC1 it will always send out a broadcast out every port except for the port the request came in on.

Switch2 will also broadcast out the request to all ports except the one it came in on.

2.) Now that PC2 has pinged PC5 the switches will have a MAC table of PC1,PC2,PC4 and PC5 MAC addresses. Here’s the command below to verify.

If PC1 pinged PC5 should or shouldn’t it (PC1) need to send out an ARP request?

Answer: PC1 will still need to use ARP even if the switch knows where the MAC address is because what is ARP? ARP allows a host to find out the MAC address of another host by only knowing it’s IP address. The Host (PC1) sends out a broadcast saying “Who has this IP Address 192.168.1.5” because PC1 has never talked to PC5 yet. Switches work at layer 2 so they don’t have a clue what that means so they can’t help speed the process along.

So again there is an ARP broadcast and only PC5 will respond to it.

3.) What would happen if PC1 would ping PC6

Because of the last question this one should be a little bit more obvious. PC1 has never talked to PC6 yet. Again even if switches 1&2 knew about PC1 and PC6 address PC1 would still need to use ARP to find out PC6’s MAC address.

4.) What would happen if PC1 pinged PC3

Again PC1 would need to call on an ARP broadcast because it has never directly talked with PC3. ie. PC3 is not in the MAC table on host PC1. Much like 2 & 3, we’re going to need to call on our friend ARP to get PC1 to be able to talk directly to PC3.

If PC1 wanted to ping any other device it’s already talked to it would talk directly to it without the need of sending out an ARP broadcast!